Low energy electron microscopy

ABSTRACT

The disclosure relates to a low energy electron microscopy. The electron microscopy includes a vacuum chamber; an electron gun used to emit electron beam; a diffraction chamber; an imaging device; a sample holder used to fix two-dimensional nanomaterial sample; a vacuum pumping device; and a control computer. The electron beam transmits the sample to form a transmission electron beam and diffraction electron beam. The control computer includes a switching module to switch the work mode between a large beam spot diffraction imaging mode and small beam spot diffraction imaging mode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims all benefits accruing under 35 U.S.C. § 119 fromChina Patent Application No. 201610404782.4, filed on Jun. 8, 2016 inthe China Intellectual Property Office, the disclosure of which isincorporated herein by reference.

BACKGROUND 1. Technical Field

The present disclosure relates to a transmission-type low energyelectron microscopy and a transmitted electron diffraction method tocharacterize the large area two-dimensional nanomaterial sample.

2. Description of Related Art

Graphene has attracted great interest owing to its unique properties andpotential applications. A requirement for high-end applications ofgraphene, particularly in electronics and photonics, is the completecontrol over the structure of the material, i.e., lateral size, layerthickness homogeneity, and purity. Thus, wafer-scale single crystalgraphene is highly sought in these years.

At present, single crystal graphene domains, from millimeter-sized tocentimeter-sized, can be synthesized by CVD. The most accurate anddecisive method to characterize the crystalline nature of a graphenedomain is electron diffraction, such as low energy electron diffraction(LEED) and selected area electron diffraction (SAED). Usually, somesample points were selected uniformly in a specific area, and theLEED/SAED patterns at the points were collected. The crystaldistribution is given by comparing these LEED/SAED patterns. LEEDpattern comes from the backscattered electrons, which will include asignal from the substrate beneath the graphene. The small beam size ofLEED also limits the characterization efficiency. SAED pattern intransmission electron microscope (TEM) comes from the transmittedelectrons at high energy. Since large magnification and high resolutionare required for the modern TEM development, the size of SAED apertureis usually from nanometer size to micrometer size. It is time-consumingto map the crystal distribution of one graphene domain at evenmillimeter size. Besides that, sample larger than 3 millimeters cannotbe entirely transferred onto the TEM grid because the holder size isfixed. It is necessary to develop an efficient method to characterizethe crystal distribution of large area sample.

What is needed, therefore, is a transmission-type low energy electronmicroscopy that overcomes the problems as discussed above.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the embodiments can be better understood with referenceto the following drawings. The components in the drawings are notnecessarily drawn to scale, the emphasis instead being placed uponclearly illustrating the principles of the embodiments. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 is a schematic section view of one embodiment of atransmission-type low energy electron microscopy.

FIG. 2 is a schematic section view of electron diffraction andtransmission of the transmission-type low energy electron microscopy.

FIG. 3 is a schematic section view of one embodiment of a vacuum pumpingdevice of the transmission-type low energy electron microscopy.

FIG. 4 is a schematic diagram of electron diffraction when electron beampasses through two-dimensional (2D) nanomaterial or three-dimensional(3D) nanomaterial.

FIG. 5 shows optical images of graphene island sample of example 1.

FIG. 6 shows transmission and diffraction images when electron beampasses through a graphene island I of FIG. 5 and schematic illustrationsof transmission and diffraction images.

FIG. 7 shows transmission and diffraction images when electron beampasses through a graphene island II of FIG. 5 and schematicillustrations of transmission and diffraction images.

FIG. 8 shows electron diffraction patterns corresponding to fourdifferent regions in graphene island I of FIG. 5.

FIG. 9 shows electron diffraction patterns corresponding to fourdifferent regions in graphene island II of FIG. 5.

FIG. 10 shows transmitted electron diffraction and imaging of a largearea continuous polycrystalline graphene of example 2.

FIG. 11 shows transmitted electron diffraction and imaging of MoS₂ ofexample 3.

FIG. 12 shows transmitted electron diffraction pattern of watermolecular adsorbed on a single crystal graphene of example 4 fromappearing to disappearing, wherein exposure time was fixed at 15 s foreach image.

FIG. 13 shows transmitted electron diffraction patterns of watermolecular adsorbed on a polycrystalline graphene of example 5 withdifferent crystal orientations.

FIG. 14 shows transmitted electron diffraction patterns of apolycrystalline graphene of example 6.

FIG. 15 shows a series of diffraction images obtained by moving theelectron gun radiated on the CGF sample of example 7, wherein the arrowsindicate the scanning directions, up and down first and then right toleft.

FIG. 16 shows sin θ as a function of the wavelength of acceleratedelectrons, wherein θ1 is the diffraction angle corresponding to thegraphene crystal face (10-10), and θ2 is the diffraction anglecorresponding to the graphene crystal face (11-20).

DETAILED DESCRIPTION

It will be appreciated that for simplicity and clarity of illustration,where appropriate, reference numerals have been repeated among thedifferent figures to indicate corresponding or analogous elements. Inaddition, numerous specific details are set forth in order to provide athorough understanding of the embodiments described herein. However, itwill be understood by those of ordinary skill in the art that theembodiments described herein can be practiced without these specificdetails. In other instances, methods, procedures, and components havenot been described in detail so as not to obscure the related relevantfeature being described. The drawings are not necessarily to scale, andthe proportions of certain parts may be exaggerated to better illustratedetails and features. The description is not to considered as limitingthe scope of the embodiments described herein.

Several definitions that apply throughout this disclosure will now bepresented.

The connection can be such that the objects are permanently connected orreleasably connected. The term “substantially” is defined to essentiallyconforming to the particular dimension, shape or other word thatsubstantially modifies, such that the component need not be exact. Forexample, substantially cylindrical means that the object resembles acylinder, but can have one or more deviations from a true cylinder. Theterm “comprising” means “including, but not necessarily limited to”; itspecifically indicates open-ended inclusion or membership in aso-described combination, group, series and the like. It should be notedthat references to “an” or “one” embodiment in this disclosure are notnecessarily to the same embodiment, and such references mean at leastone. In general, the word “module,” as used herein, refers to logicembodied in hardware or firmware, or to a collection of softwareinstructions, written in a programming language, such as, for example,Java, C, or assembly. One or more software instructions in the modulesmay be embedded in firmware, such as an EPROM. It will be appreciatedthat modules may comprise connected logic units, such as gates andflip-flops, and may comprise programmable units, such as programmablegate arrays or processors. The modules described herein may beimplemented as either software and/or hardware modules and may be storedin any type of computer-readable medium or other computer storagedevice.

References will now be made to the drawings to describe, in detail,various embodiments of the present transmission-type low energy electronmicroscopy and method for characterizing two-dimensional nanomaterial.This disclosure provides a transmitted electron diffraction method tocharacterize the centimeter-sized graphene domain at relatively lowenergy. The method has adopted an experimental scheme similar to that ofThomson which has demonstrated the wave nature of electrons. With avariable beam size from hundreds of micrometers to half a centimeter,transmitted electron diffraction and imaging of large area graphenesample can be easily observed to verify the crystal texture of largearea sample. The relative crystal orientation in a different area ischaracterized in one test. The crystal distribution of polycrystallineMoS₂ has also been analyzed. With the low energy electron beam, a 2×2 0°adsorption pattern of water on suspended graphene is also observed.

Referring to FIGS. 1-2, a transmission-type low energy electronmicroscopy 10 of one embodiment is provided. The electron microscopy 10comprises a vacuum chamber 11, an electron gun 12, a diffraction chamber13, a sample holder 14, a core column 15, a vacuum pumping device 16,and a control computer 17.

The electron gun 12 is located in the vacuum chamber 11 and used to emitelectron beam. The diffraction chamber 13 is in communication with thevacuum chamber 11. The sample holder 14 is used to fix a two-dimensionalnanomaterial sample 20. The sample holder 14 can be located at the jointbetween the vacuum chamber 11 and the diffraction chamber 13. Thus, theelectron beam emitted from the electron gun 12 can pass through thetwo-dimensional nanomaterial sample 20 and enter the diffraction chamber13. The core column 15 is communicated to the diffraction chamber 13.The vacuum pumping device 16 is communicated to the vacuum chamber 11.The control computer 17 is used to control the work of the electronmicroscopy 10.

An imaging device 132 and an anode 134 is located in the diffractionchamber 13. The imaging device 132 is located between the electron gun12 and the anode 134. The electron beam emitted from the electron gun 12would move to the imaging device 132 under the force of anode 134. Theelectron beam emitted from the electron gun 12 would pass through thetwo-dimensional nanomaterial sample 20 and reach the imaging device 132to form diffraction spot and/or diffraction imaging. The diffractionspot and/or diffraction imaging can be used to analysis the structure ofthe two-dimensional nanomaterial sample 20. The imaging device 132 canbe a fluorescent screen configured to directly show the diffraction spotand/or diffraction imaging or a charge coupled device (CCD) to acquireand send the diffraction spot and/or diffraction imaging to the controlcomputer 17.

The electron beam emitted from the electron gun 12 can have an energy ina range from about 800 eV to about 3000 eV, a current in a range fromabout 0.1 microampere to about 1 microampere, and a spot diameter in arange from about 100 micrometers to about 1 centimeter. The electron gun12 can include a hot cathode electron source or a field emission coldcathode electron source. As shown in FIG. 2, in one embodiment, theelectron gun 12 is a pre-focusing multiple lens beam electron gun usinga quadrupole electrostatic focusing system. The electron gun 12 includesa cathode electron emitter C and four focusing electrodes A1, A2, A3,and A4. The four focusing electrodes are used to control the spotdiameter of the electron beam. The electron gun 12 can also be a laminargun. The laminar gun can have a more uniform spot and greater currentdensity to improve the imaging diffraction quality. Furthermore, theelectron microscopy 10 can include a moving platform 19 configured tomove the electron gun 12 to scan the two-dimensional nanomaterial sample20.

The sample holder 14 can have any structure and size as long as it canbe used to fix the two-dimensional nanomaterial sample 20. In oneembodiment, the sample holder 14 is a round copper plate having a roundthrough hole in the middle of the plate. The diameter of the throughhole is less than the size of the two-dimensional nanomaterial sample 20so that the two-dimensional nanomaterial sample 20 can cover the throughhole. The sample holder 14 can further include a moving device so thatthe two-dimensional nanomaterial sample 20 can be moved along XYZdirections and scanned by the electron beam.

The sample holder 14 can further include a heating element to heat thetwo-dimensional nanomaterial sample 20. Thus, the structure andinteraction of the two-dimensional nanomaterial sample 20 under varioustemperature can be observed. The sample holder 14 can be heated to atemperature in a range from room temperature to about 1500K. In oneembodiment, sample holder 14 includes tow electrodes 142 spaced fromeach other. The two-dimensional nanomaterial sample 20 is fixed on asupporter such as a copper mesh or a carbon nanotube film. Then, thesupporter is fixed on and electrically connected to the two electrodes142. The tow electrodes 142 can be used to apply a current to thesupporter to heat the two-dimensional nanomaterial sample 20. The sampleholder 14 can further include a temperature sensor to detect thetemperature of the two-dimensional nanomaterial sample 20.

Furthermore, the electron microscopy 10 can include a sprayer 18. Thesprayer 18 is adjacent to the sample holder 14 so that the materialspray out of the sprayer 18 can be attached to the two-dimensionalnanomaterial sample 20. Thus, the absorption or reaction between thetwo-dimensional nanomaterial sample 20 and other materials can beobserved.

Furthermore, the electron microscopy 10 can include a conductive rod 29having a first end and a second end opposite to the first end. The firstend of the conductive rod 29 is fixed on the inner wall of thediffraction chamber 13. The conductive rod 29 is rotatable and can berotated to be in front of the imaging device 132 to shield thezero-order diffraction spot or transmission spot. Thus, only diffractionimage can be obtained by the imaging device 132. The electron microscopy10 can also include a Faraday cup (not shown) so that only a singlediffraction beam can be obtained from the imaging device 132.

Referring to FIG. 3, the vacuum pumping device 16 of one embodimentincludes an ion pump 161, a first molecular pump 162, a second molecularpump 163, a mechanical pump 164, and a control unit 165. The ion pump161 and the second molecular pump 163 are respectively connected to thevacuum chamber 11. The first molecular pump 162 is connected to thevacuum chamber 11 via a pre-vacuum chamber 166. The mechanical pump 164is respectively connected to the first molecular pump 162 and the secondmolecular pump 163. The control unit 165 is configured to control thework of the vacuum pumping device 16. The pressure of the vacuum chamber11 can be kept at a range from about 10⁻³ Pa to about 10⁻⁶ Pa. Thediffraction spots and diffraction image of transmission electron can beobserved in the pressure range.

The control computer 17 includes a switching module, a calculatingmodule, an image processing module, and a distance controlling module.The switching module is configured to switch the work of the electronmicroscopy 10 between large beam spot diffraction imaging mode and smallbeam spot diffraction imaging mode. In the large beam spot imaging mode,the electron beam is larger than the two-dimensional nanomaterial sample20 and used to irradiate the entire surface of the two-dimensionalnanomaterial sample 20 so that the diffraction imaging of the entiretwo-dimensional nanomaterial sample 20 is obtained. In the small beamspot imaging mode, the electron beam is smaller than the two-dimensionalnanomaterial sample 20 and used to irradiate partial surface or scan theentire surface of the two-dimensional nanomaterial sample 20 so that thediffraction imaging of part of the two-dimensional nanomaterial sample20 is obtained. The calculating module is configured to calculate thelattice period of the two-dimensional nanomaterial sample 20 asdescribed below. The image processing module is configured to processthe diffraction imaging, such as obtain radius R of diffraction ring.The distance controlling module is configured to adjust the distance Dbetween the two-dimensional nanomaterial sample 20 and the imagingdevice 132.

The electron microscopy 10 is beneficial for two-dimensionalnanomaterial, especially, two-dimensional nanomaterial only having asingle layer of atoms. The difference between the conventional electrondiffraction of three-dimensional nanomaterial and the electrondiffraction of two-dimensional nanomaterial is described below.

Referring to FIG. 4 (a), the electron diffraction of the two-dimensionalnanomaterial satisfies the condition d sin θ=λ, wherein d represents thelattice period of the two-dimensional nanomaterial, θ represents theangle between the diffraction electron beam 24 and the transmissionelectron beam 26. Referring to FIG. 4 (b), the electron diffraction ofthe three-dimensional nanomaterial satisfies the condition 2d′ sin θ′=λ,wherein d′ represents the interplanar spacing of the three-dimensionalnanomaterial, θ′ represents the angle between the incident electron beam22 and the crystal surface 28 of the three-dimensional nanomaterial. Inthe conventional electron diffraction of three-dimensional nanomaterial,the angle between the diffraction electron beam 24 and the transmissionelectron beam 26 is 2θ′. Usually, in selected area electron diffraction,the θ or θ′ is much small and satisfies the condition θ≅sin θ≅tan θ orθ′≅sin θ′≅tan θ′. Thus, in the electron diffraction of thetwo-dimensional nanomaterial, it satisfies the condition d sin θ≅dθ=λ,however, in the conventional electron diffraction of three-dimensionalnanomaterial, it satisfies the condition 2d′ sin θ′≅2d′θ′=d′2θ′=λ.

The calculating module of the control computer 17 is configured tocalculate the lattice period d of the two-dimensional nanomaterialsample 20 according to the formula d sin θ≅dθ=λ. Referring to FIG. 2,along the same crystal direction, the diffraction electron beam 24 forma diffraction ring on the imaging device 132, and the transmissionelectron beam 26 form a transmission spot on the imaging device 132. Thedistance between the diffraction ring and the transmission spot is equalto the n radius R of diffraction ring and can be obtained by thedistance controlling module of the control computer 17. The distance Dbetween the two-dimensional nanomaterial sample 20 and the imagingdevice 132 can be obtained by the distance controlling module. The θ canbe calculated by the radius R and distance D. The wavelength λ can beobtained from the energy of the incident electron beam 22. Thus, thelattice period d of the two-dimensional nanomaterial sample 20 can becalculated according to the formulad sin θ≅dθ=λ.

In both selected area electron diffraction and micro-diffraction, thetransmission-type electron microscopy uses parallel or nearly parallelelectron beam. In selected area electron diffraction, the diameter ofthe electron beam is in a range from about 0.5 micrometers to about 1micrometer. In micro-diffraction, the diameter of the electron beam isless than 0.5 micrometers. In conventional electron diffractometer,electron beam smaller than the sample is used, and only parts of thesample are diffracted. LEED can have low energy electron microscopy(LEEM) mode, but the LEEM mode can only select one diffraction beam toform an image. The transmission-type electron microscopy 10 can be usedto observe the two-dimensional nanomaterial sample 20 such as a singlelayer graphene, multi-layers graphene or MoS₂. The two-dimensionalnanomaterial sample 20 can have a size in a range from about 10micrometers to about several millimeters can be observed entirely. Thetwo-dimensional nanomaterial sample 20 can have a size greater than 1centimeter can be rapidly scanned by moving the two-dimensionalnanomaterial sample 20. The energy of the electron beam of thetransmission-type electron microscopy 10 is lower and would not destroythe two-dimensional nanomaterial sample 20. The two-dimensionalnanomaterial sample 20 can be suspended by the sample holder 14 andprevented from being affected by the substrate. The examples ofobserving the graphene or MoS₂ using the transmission-type electronmicroscopy 10 are provided below.

Example 1

Referring to FIGS. 5a-5b , in example 1, the two-dimensionalnanomaterial sample 20 is graphene islands grown on a copper foil. FIG.5a and FIG. 5b show graphene island grown on a copper foil by CVDmethod, and FIG. 5c shows a CGF (CNT/graphene hybrid film) includinggraphene islands located on crossed and stacked carbon nanotube films.Two graphene islands having an area greater than 1 square millimetersare marked respectively as numbers I and II. Referring to FIG. 5c , thetwo graphene islands respectively marked as numbers I and II aretransferred from the copper foil to two layers of cross-stackingsuper-aligned drawn carbon nanotube film to form a CGF. The drawn carbonnanotube film includes a plurality of carbon nanotubes orderly arrangedand spaced from each other. Thus, parts of the two graphene islands aresuspended on a hole between adjacent carbon nanotubes. The drawn carbonnanotube film having the two graphene islands thereon is located on theround copper plate and covers the round through hole of the round copperplate. The drawn carbon nanotube film is an ultra thin, sparse porousstructure and has little effect on the two-dimensional nanomaterialsample 20. Furthermore, because the primary diffraction spot of thedrawn carbon nanotube film happens between adjacent wall of the carbonnanotubes, has a low angle and would not influence the diffraction spotsof the two-dimensional nanomaterial sample 20.

The two graphene islands are observed by the transmission-type electronmicroscopy 10. When the electron beam irradiates entire graphene islandI, the central transmission image and diffraction image of grapheneisland I can be observed as shown in FIGS. 6a and 6b , respectively. Ascan be seen from FIGS. 6a and 6b , the shape of the transmission patternand diffraction pattern correspond to that of graphene island I. FIGS.6c and 6d show the schematics of transmission image and diffractionimage for a single crystal graphene domain, respectively, which canillustrate FIGS. 6a and 6b well. Thus, the graphene island I is singlelayer graphene.

When the electron beam covers the whole graphene island II, thetransmission image of graphene island II can be observed as shown inFIG. 7a similar to FIG. 6a but the diffraction image becomes a complexpattern as shown in FIG. 7b . FIGS. 7c and 7d show the schematics oftransmission image and diffraction image for a graphene island includingthree single crystal graphene domains, helping us to understand FIG. 7b. Thus, the graphene island II includes three layer graphenes.

Since electrons passing through different graphene domains will bediffracted by different azimuthal angles, the diffraction image is asuperposition of diffraction patterns corresponding to each graphenedomain. Thus, the graphene island II cannot be recognized from itsdiffraction image. When the electron beam is focused, the diffractionpattern like selected area electron diffraction pattern can be achievedfor both graphene island I and graphene island II. FIG. 8 shows fourdiffraction patterns of four areas of the graphene island I. FIG. 9shows four diffraction patterns of four areas of the graphene island II.The diffraction patterns of graphene island I in FIG. 8 show the sameset of hexagonal diffraction spots, confirming that graphene island I isa single crystal. In contrast, the diffraction patterns of grapheneisland II show three set of hexagonal diffraction spots number as I, IIand III in FIG. 9, indicating that graphene island II is composed of atleast three single crystal graphene domains.

Example 2

Referring to FIG. 10a , in example 2, the two-dimensional nanomaterialsample 20 is a large area continuous polycrystalline graphenetransferred onto a silicon substrate with a 300 nanometers SiO₂ layer.The optical contrast of graphene itself is homogeneous, indicatinguniform single layer graphene with few double layer graphene islands.FIG. 10b shows the selected area electron diffraction pattern,confirming the graphene to be a single layer. The electron diffractionimage is shown in FIG. 10c . Different from FIG. 6b , two sets ofdiffraction patterns with the same shape as the transmission spot appearin FIG. 10c . When the electron beam is focused, the diffraction patterncomposed of two sets of hexagonal diffraction spots can be obtained, asshown in FIG. 10d . Since the graphene sample was verified to be singlelayer via optical of FIG. 10a , it is believed that FIGS. 10c and 10dreveal the crystal distribution of the whole graphene of millimetersize. Because the graphene is polycrystalline graphene composed of smallsingle crystal graphene domains of micrometer size, the diffractionpattern of mm² polycrystalline graphene is a superposition of thousandsof small graphene domains. Two sets of diffraction patterns indicatethat the graphene has two preferred crystal orientations.

Example 3

Referring to FIG. 11a , in example 3, the two-dimensional nanomaterialsample 20 is MoS₂ synthesized on a silicon substrate with oxidationlayer. The darker flakes in FIG. 11a are MoS₂ with 19% coverage rate.The diffraction pattern as shown in FIG. 11b indicates that the MoS₂flakes are polycrystalline. Many sets of diffraction spots come out inthe image, showing none preferred orientation. However, when theelectron beam covers few MoS₂ flakes, the orientations of these MoS₂flakes can be clearly seen. FIG. 11c shows the transmitted electrondiffraction pattern of MoS₂ with two crystal orientations, and FIG. 11dshows the transmitted electron diffraction pattern of MoS₂ with only onecrystal orientation. The size of single MoS₂ flake is about tens ofmicrometers and much smaller than the beam size. Thus, the resolution ofour low energy electron transmission diffraction can exceed its beamsize limit of the transmission-type electron microscopy 10.

Example 4

The two-dimensional nanomaterial sample 20 of example 4 is the same asthe single crystal graphene of the example 1. In example 4, the singlecrystal graphene is washed by deionized water before observation on thetransmission-type electron microscopy 10. FIG. 12a shows that a set ofhexagonal diffraction spots appeared near to the transmission spot asshown. The rotation angle of these diffraction spots was the same as{10-10} diffraction spots of graphene. However, after a few seconds,these extra diffraction spots gradually disappeared as shown in FIGS.12b, 12c, and 12d . When the electron gun is moved to a new position,the same diffraction pattern could appear and disappear again. The extradiffraction spots should correspond to some metastable phenomena.Adsorption/desorption is one of the most possible cause for thisphenomenon. When the CGF is heated to incandescence in a vacuum, and itis found that no extra diffraction spots existed. Since the CGF wasrinsed in deionized water in preparation, it is assumed that watermolecule was the most possible adsorption species. For the CGF sampleafter electron irradiation or being heated, water mist has been sprayedon the sample for two minutes. Then the extra diffraction patterncorresponding to the same region appeared again. As well as before, thisdiffraction pattern disappeared due to electron irradiation. Based onthe contrast experiment above, it is believed that the extra pattern iscaused by the adsorption of water molecules on graphene. The in-planelattice spacing of this adsorption crystal lattice was calculated to betwice the {10-10} in-plane lattice spacing of graphene, and the crystalorientation of the adsorption crystal lattice was the same as graphene.It is called 2×2 0° adsorption pattern. Beyond the adsorptiondiffraction spots, adsorption diffraction image is also obtained whenelectron beam was enlarged. The adsorption image shows the same shapeand rotation angle of the diffraction image of graphene, indicating thatthe adsorbed water molecules are highly correlated with the graphenelattice on a large scale. Besides single crystal graphene, thediffraction pattern of adsorption water can also be seen inpolycrystalline graphene.

Example 5

The two-dimensional nanomaterial sample 20 of example 4 is the same asthe polycrystalline graphene of example 1. In example 5, thepolycrystalline graphene is washed by deionized water before observationon the transmission-type electron microscopy 10. FIGS. 13a, 13b, and 13cshow that the adsorption patterns of polycrystalline graphene with one,two, and three main crystal orientations, respectively. It is found thatFIG. 13a has only one main crystal orientation, FIG. 13b has two maincrystal orientations, and FIG. 13c has three main crystal orientations.In each of them, the rotation angle of adsorption diffraction spotsvaries along with the crystal orientation of graphene.

Example 6

The two-dimensional nanomaterial sample 20 of example 6 is the same asthe large area continuous polycrystalline graphene of example 2. Inexample 5, the large area continuous polycrystalline graphene istransferred to two super-aligned drawn carbon nanotube films to form aCGF. FIGS. 14a and 14b are diffraction patterns of the same region ofpolycrystalline graphene, wherein FIG. 14a is obtained before rotatingthe polycrystalline graphene, and FIG. 14b is obtained after thepolycrystalline graphene is rotated 90°. The sample in FIG. 14b wasrotated 90°, relative to FIG. 14a . FIGS. 14a and 14b show that when theCGF sample was rotated 90°, the diffraction pattern of polycrystallinegraphene is rotated 90°.

Example 7

The two-dimensional nanomaterial sample 20 of example 7 is the same asthe large area continuous polycrystalline graphene of example 6. Inexample 7, 1 millimeter narrow slit was made by laser etching on the CGFsample before observation on the transmission-type electron microscopy10. When the electron gun is moved, a series of diffraction images asshown in FIG. 15 are obtained. When the electron beam passed through thenarrow slit, diffraction spots became dim and disappeared, indicatingthat the diffraction spots arise from the CGF sample.

Example 8

The two-dimensional nanomaterial sample 20 of example 8 is the same asthe single crystal graphene of example 1. In example 8, the singlecrystal graphene is transferred to super-aligned drawn carbon nanotubefilms to form a CGF. When the acceleration voltage is changed, thedistance from the diffraction spots and rings to the transmission spotwill change. If the camera length and the distance are measured from thetransmission spot to the diffraction spot, the diffraction angle θ canbe calculated. According to the theory of De Broglie, the wavelength ofthe accelerated electrons can be calculated. By doing that sin θ versusthe wavelength of the electrons A is plotted as the acceleration voltagechanged in FIG. 16. θ1 is the diffraction angle corresponding to thegraphene crystal face (10-10), and θ2 is the diffraction anglecorresponding to the graphene crystal face (11-20). Then thecorresponding in-plane lattice spacing of (10-10) face and (11-20) faceis calculated by linear fitting the data according to formula d sin θ=λ.The in-plane lattice spacing d1 corresponding to (10-10) face wascalculated to be 0.213 nanometers, and the in-plane lattice spacing d2corresponding to (11-20) face was calculated to be 0.126 nanometersaccording to the formula d=λ/sin θ. Compared with the theoretical valuesof the in-plane lattice spacings of graphene, our experiment values areacceptable. As well as graphene, the in-plane lattice spacing of MoS₂can also be calculated through this method.

It is to be understood that the above-described embodiments are intendedto illustrate rather than limit the disclosure. Any elements describedin accordance with any embodiments is understood that they can be usedin addition or substituted in other embodiments. Embodiments can also beused together. Variations may be made to the embodiments withoutdeparting from the spirit of the disclosure. The above-describedembodiments illustrate the scope of the disclosure but do not restrictthe scope of the disclosure.

Depending on the embodiment, certain of the steps of methods describedmay be removed, others may be added, and the sequence of steps may bealtered. It is also to be understood that the description and the claimsdrawn to a method may include some indication in reference to certainsteps. However, the indication used is only to be viewed foridentification purposes and not as a suggestion as to an order for thesteps.

What is claimed is:
 1. A transmission-type low energy electronmicroscopy, comprising: a vacuum chamber; an electron gun located in thevacuum chamber and configured to emit an electron beam; a diffractionchamber in communication with the vacuum chamber; an imaging devicelocated in the diffraction chamber; a core column connected to thediffraction chamber; a vacuum pumping device connected to the vacuumchamber; a sample holder configured to fix a two-dimensionalnanomaterial sample so that the electron beam passes through thetwo-dimensional nanomaterial sample and enters the diffraction chamberto form a diffraction electron beam and a transmission electron beam toform an image on the imaging device; and a control computer, wherein thecontrol computer comprises a switching module configured to switch thetransmission-type low energy electron microscopy between a large beamspot diffraction imaging mode and a small beam spot diffraction imagingmode; in the large beam spot diffraction imaging mode, the electron beamis larger than the two-dimensional nanomaterial sample and used toirradiate entire surface of the two-dimensional nanomaterial sample sothat a first diffraction imaging of the entire two-dimensionalnanomaterial sample is obtained; and in the small beam spot diffractionimaging mode, the electron beam is smaller than the two-dimensionalnanomaterial sample and used to irradiate partial surface or scan entiresurface of the two-dimensional nanomaterial sample so that a seconddiffraction imaging, of part of the two-dimensional nanomaterial sample,is obtained.
 2. The transmission-type low energy electron microscopy ofclaim 1, wherein the control computer further comprises an imageprocessing module and a distance controlling module; the imageprocessing module is configured to process the first diffraction imagingor the second diffraction imaging to obtain a radius R of diffractionrings, and the distance controlling module is configured to adjust adistance D between the two-dimensional nanomaterial sample and theimaging device.
 3. The transmission-type low energy electron microscopyof claim 2, wherein the control computer further comprises a calculatingmodule; and the calculating module is configured to calculate an angle θbetween the diffraction electron beam and the transmission electron beamby the radius R and the distance D, and further calculate a latticeperiod d of the two-dimensional nanomaterial sample according to aformula d sin θ≅dθ=λ, where λ represents a wavelength of the electronbeam.
 4. The transmission-type low energy electron microscopy of claim1, wherein the imaging device is a fluorescent screen.
 5. Thetransmission-type low energy electron microscopy of claim 1, wherein theimaging device is a charge coupled device.
 6. The transmission-type lowenergy electron microscopy of claim 1, wherein the electron beam has anenergy in a range from about 800 eV to about 3000 eV, a current in arange from about 0.1 microamperes to about 1 microampere, and a spotdiameter in a range from about 100 micrometers to about 1 centimeter. 7.The transmission-type low energy electron microscopy of claim 1, furthercomprising a sprayer configured to spray material to the two-dimensionalnanomaterial sample.
 8. The transmission-type low energy electronmicroscopy of claim 1, further comprising a conductive rod having afirst end fixed on an inner wall of the diffraction chamber, and theconductive rod is rotatable around the first end and can be rotated tobe in front of the imaging device to shield a zero-order diffractionspot or transmission spot.
 9. The transmission-type low energy electronmicroscopy of claim 1, further comprising a heating element to heat thetwo-dimensional nanomaterial sample and a temperature sensor to detect atemperature of the two-dimensional nanomaterial sample.
 10. Thetransmission-type low energy electron microscopy of claim 1, furthercomprising a moving platform configured to move the electron gun to scanthe two-dimensional nanomaterial sample.